Partitioning of semisynthetic lipidated N-Ras in lipid raft nanodomains determined by FRET to lipid domain markers

Abstract

Cellular membranes are heterogeneous planar lipid bilayers displaying lateral phase separation with the nanometer-scale liquid-ordered phase (aka \"lipid rafts\" or Lo) surrounded by the liquid-disordered phase (Ld). Many membrane-associated proteins were found to stably integrate in the rafts, which is critical for their biological function. Isoforms H and N of Ras GTPase possess a unique ability to switch their lipid domain preference depending on the type of bound guanine nucleotide (GDP or GTP). This behavior, however, has never been reproduced in vitro in model bilayers with recombinant proteins, and therefore has been attributed to action of other proteins binding Ras at the membrane surface. In this paper, we report the observation of the nucleotide-dependent switch of lipid domain preferences of the semisynthetic lipidated N-Ras in raft lipid vesicles in the absence of other proteins. To detect segregation of Ras molecules in raft and disordered lipid domains, we measured Forster Resonance Energy Transfer (FRET) between the donor fluorophore, mant, attached to the protein-bound guanine nucleotides, and the acceptor, rhodamine-conjugated lipid, localized to the liquid-disordered domains. We demonstrated that N-Ras preferentially populated raft domains when bound to mant-GDP, while losing preference for rafts when it was associated with a GTP mimic, mant-GppNHp. At the same time, the isolated lipidated C-terminal peptide of N-Ras was found localized outside of the liquid-ordered rafts, most likely--in the bulk disordered lipid.